Pros
I got great experiences to put on my resume. As soon as I showed my competency I was put in charge of everything and anything needed. I went from being recruited to recruiting, training and supervising new interns in a matter of weeks, planning and organizing events, being the spokesperson for our campaign to newspapers, radio, and television, and generally being the organizers right hand (wo)man. These are experiences that just don't happen in a paid position without months or years of work leading up to it.
Cons
The organizers were over zealous and at the end of the campaign I felt taken advantage of. We were running a campaign on a college campus and they would obviously judge volunteers and interns who chose not to volunteer every waking second of their lives. I literally heard a volunteer say they had to cancel their shift because they had a big paper they had to write for class and the organizer responded with the words, "Well, if that's where your priorities are." Those words on their own are benign enough, yet they were said in a way that obviously judged the student for not choosing to volunteer. At the time I devoted myself completely because I thought the cause was just that important, that we were helping to save the world. Afterwords I wondered why I had let my life get so consumed.